The present invention relates to a hydrogen storage material and a process for producing the same. More particularly, it relates to a hydrogen storage material which is, while with a minimized cobalt content, excellent in insusceptibility to grain size reduction and hydrogen storage characteristics (PCT characteristics) and exhibits not only excellent initial activity that is an important characteristic for use in a battery but high discharge characteristics for use in electric tools or low-temperature characteristics for use in hybrid electric vehicles, and a process for producing the same.
Nickel-hydrogen storage batteries (secondary batteries) having a hydrogen storage material in the anode have recently been attracting attention as high capacity alkaline storage batteries taking the place of nickel-cadmium storage batteries. The hydrogen storage materials that are currently used widely are composed of five elements, i.e., Mm (misch metal), Ni, Al, Mn, and Co.
Compared with La-based alloys, the Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloys enable constructing an anode out of relatively cheap materials and provide closed nickel-hydrogen storage batteries having a long cycle life and a suppressed inner pressure rise which is caused by gas generated in case of an overcharge and have therefore been used widely as an electrode material.
The Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloys currently used are designed to have a prolonged cycle life by preventing the alloys from reducing their grain size. It is generally known that about 10% by weight of Co (0.6 to 1.0 in an atomic ratio) is required to prevent the grain size reduction of the alloy. It is also accepted that a given amount of Co is necessary for securing excellent hydrogen storage characteristics and anticorrosion.
However, the material cost increases with the Co content, which is problematical from the aspect of material cost. Taking application of the hydrogen storage material to large batteries into consideration, such as the power source of electric vehicles, and the ever expanding market of nickel-hydrogen storage batteries, in particular, the material cost is weighty in choosing anode materials and has been a matter of concern.
To settle the above problem, Japanese Patent Application Laid-Open No. 213319/97 proposes altering the composition of the Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloy and adding thereto a small amount of an additional element. Use of the hydrogen storage material powder disclosed therein as an anode makes it feasible to reduce the Co content and yet to suppress deterioration of the anode caused by the alloy""s reduction in grain size below a certain level and thereby to extent the cycle life of the battery.
Because the alloy of the composition disclosed in the 213319/97 does not always secure stability in its characteristics, the present inventors have proposed in Japanese Patent Application Lain-Open No. 152533/99 a composition and a production process for obtaining satisfactory initial activity, whereby a low-Co alloy has now come to be used in special applications.
However, where the hydrogen storage materials disclosed in the above publications (Laid-Open No. 213319/97 and Laid-Open No. 152533/99 are used, the batteries have insufficient discharge characteristics particularly in low temperature and cannot be used for electric tools needing high discharge characteristics or for hybrid electric vehicles.
Accordingly, an object of the present invention is to provide a hydrogen storage material of which the production cost is reduced by extremely decreasing its cobalt content and which exhibits excellent insusceptibility to grain size reduction, excellent hydrogen storage characteristics, satisfactory discharge characteristics, and satisfactory initial activation and a process for producing the same.
As a result of extensive studies, the present inventors have found that the above object is accomplished by a hydrogen storage material of AB5 structure having a specific stoichiometric composition (B site rich), particularly a composition of 4.0 less than Nixe2x89xa64.3 and 0.25xe2x89xa6Mnxe2x89xa60.4, and the c-axis of which is in a given range. They have also found that such a
hydrogen storage material is obtainable with the above-described specific composition when a casting temperature and heat treating conditions satisfy a given relationship.
The present invention has been reached based on the above findings and provides a hydrogen storage material which is an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure represented by general formula:
MmNiaMnbAlcCod
wherein Mm denotes a misch metal, 4.0 less than axe2x89xa64.3, 0.25xe2x89xa6bxe2x89xa60.4, 0.25xe2x89xa6cxe2x89xa60.4, 0.3xe2x89xa6dxe2x89xa60.5, and 5.05xe2x89xa6a+b+c+dxe2x89xa65.25, or general formula:
MmNiaMnbAlcCodXe
wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0 less than axe2x89xa64.3, 0.25xe2x89xa6bxe2x89xa60.4, 0.25xe2x89xa6cxe2x89xa60.4, 0.3xe2x89xa6dxe2x89xa60.5, 0 less than exe2x89xa60.1, and 5.05xe2x89xa6a+b+c+d+exe2x89xa65.25,
characterized in that the lattice length on the c-axis is 404.9 pm or more.
The present invention also provides a preferred process for producing the hydrogen storage material of the present invention which comprises heat-melting raw materials of a hydrogen storage material, casting the melt, and heat treating the resulting alloy in an inert gas atmosphere to produce an AB5 type hydrogen storage material having a CaCu5 type crystal structure represented by the following general formulae, characterized in that the casting temperature is 1350 to 1550xc2x0 C., the pouring temperature is 1230 to 1430xc2x0 C., and conditions of said heat treating are 1070 to 1100xc2x0 C. and 1 to 6 hours. General formula:
MmNiaMnbAlcCod
wherein Mm denotes a misch metal, 4.0 less than axe2x89xa64.3, 0.25xe2x89xa6bxe2x89xa60.4, 0.25xe2x89xa6cxe2x89xa60.4, 0.3xe2x89xa6dxe2x89xa60.5, and 5.05xe2x89xa6a+b+c+dxe2x89xa65.25, or general formula:
MmNiaMnbAlcCodXe
wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0 less than axe2x89xa64.3, 0.25xe2x89xa6bxe2x89xa60.4, 0.25xe2x89xa6cxe2x89xa60.4, 0.3xe2x89xa6dxe2x89xa60.5, 0 less than exe2x89xa60.1, and 5.05xe2x89xa6a+b+c+d+exe2x89xa65.25.
The Best Mode for Carrying out the Invention:
In the above formulae, Mm donates a misch metal, a mixture of rare earth elements such as La, Ce, Pr, Nd, and Sm. The hydrogen storage material is an AB5, type hydrogen storage alloy having a CaCu5 type crystal structure having a B site-rich nonstoichiometric composition of AB5.05 to AB5.25.
In this hydrogen storage material, the compositional ratio (atomic ratio) of NiaMnbMcCod fulfills the following relationships. The ratio of Ni: 4.0 less than axe2x89xa64.3. The ratio of Mn: 0.25xe2x89xa6bxe2x89xa60.4. The ratio of Al: 0.25xe2x89xa6cxe2x89xa60.4. The ratio of Co: 0.3xe2x89xa6dxe2x89xa60.5. (a+b+c+d) is in a range of from 5.05 to 5.25.
The compositional ratio (atomic ratio) of NiaMnbAlcCodXc (wherein X is Cu and/or Fe) satisfies the following relationships. The ratio of Ni: 4.0 less than axe2x89xa64.3. The ratio of Mn: 0.25xe2x89xa6bxe2x89xa60.4. The ratio of Al: 0.25xe2x89xa6cxe2x89xa60.4. The ratio of Co: 0.3xe2x89xa6dxe2x89xa60.5. The ratio of X: 0 less than exe2x89xa60.1. (a+b+c+d+e) is in a range of from 5.05 to 5.25.
As described above, the ratio of Ni, a, is from 4.0 to 4.3, desirably from 4.1 to 4.2. If a is less than 4.0, the discharge characteristics are not satisfactory. If it exceeds 4.3, deterioration in insusceptibility to grain size reduction or life characteristics is observed.
The ratio of Mn, b, is from 0.25 to 0.4. If b is less than 0.25. the plateau pressure increases, and the hydrogen storage capacity is reduced. If it exceeds 0.4, the alloy undergoes considerable corrosion so that the battery voltage greatly decreases during storage.
The ratio of Al, c, is from 0.25 to 0.4. If c is smaller than 0.25, the plateau pressure, which is the hydrogen release pressure of a hydrogen storage material, increases to deteriorate energy efficiency in charges and discharges. If it exceeds 0.4, the hydrogen storage capacity is reduced.
The ratio of Co, d, is 0.3 to 0.5. If d is less than 0.3, the hydrogen storage characteristics or the resistance to grain size reduction are deteriorated. If it exceeds 0.5, the ratio of Co is too high to realize cost reduction.
The ratio of X, e, is from 0 up to 0.1. If e is more than 0.1, the discharge characteristics are impaired, and the hydrogen storage capacity is reduced. (a+b+c+d) or (a+b+c+d+e) (these sums will hereinafter be sometimes referred to as x, inclusively) is from 5.05 to 5.25. If x is smaller than 5.05, the battery life or the insusceptibility to grain size reduction is ruined. If x is greater than 5.25, the hydrogen storage characteristics are reduced and, at the same time, the discharge characteristics are also deteriorated.
The hydrogen storage material of the present invention has a lattice length on the c-axis of 404.9 pm or more, preferably 404.9 to 405.8 pm. If the lattice length on the c-axis is shorter than 404.9 pm, the alloy has poor insusceptibility to grain size reduction and reduced initial activation (relative magnetization). Hydrogen storage materials whose c-axis lattice length exceeds 405.8 pm are not only difficult to produce but have greatly reduced hydrogen storage capacity.
The c-axis lattice length of the hydrogen storage material has different preferred ranges according to the value of (a+b+c+d) or (a+b+c+d+e), i.e., the value x. The value x being 5.05 or greater and smaller than 5.15, the c-axis lattice length is preferably 404.9 or greater and smaller than 405.4 pm. The value x ranging from 5.15 to 5.25, the c-axis lattice length is preferably 405.4 to 405.8 pm.
Although the lattice length on the a-axis of the hydrogen storage material of the present invention is not particularly limited, it is usually from 500.3 to 501.0 pm.
The process of producing the hydrogen storage material of the present invention is then described.
Raw materials of the hydrogen storage material are weighed to give the alloying composition described above and mixed up. The mixture is melted into a melt by means of a high frequency induction furnace based on induction heating. The melt is poured into a casting mold, for example, a mold of water cooling type at a casting temperature of 1350 to 1550xc2x0 C. to obtain a hydrogen storage material. The pouring temperature is 1200 to 1450xc2x0 C. The term xe2x80x9ccasting temperaturexe2x80x9d as used herein means the temperature of the melt in the crucible at the beginning of casting, and the term xe2x80x9cpouring temperaturexe2x80x9d means the temperature of the melt at the inlet of the casting mold (i.e., the temperature of the melt before entering the casting mold).
The resulting hydrogen storage material is heat treated in an inert gas atmosphere, for example, in argon gas under heat treating conditions of 1070 to 1100xc2x0 C. and 1 to 6 hours. The cast alloy structure usually shows fine grain boundary segregation chiefly of Mn. The heat treatment is to level the segregation by heating.
There is thus obtained a hydrogen storage material which has a reduced cobalt content and yet exhibits excellent insusceptibility to grain size reduction, excellent hydrogen storage characteristics, satisfactory discharge characteristics, and satisfactory initial activation.
The hydrogen storage material, after crushed and pulverized, is suitably used as an anode of high-discharge alkaline storage batteries. The alkaline storage batteries thus provided are satisfactory in initial activation and low-temperature high-rate characteristics, and the anode of which is prevented from deterioration due to the alloy getting finer and therefore secures a long cycle life.